This invention relates to a programmable controller, capable of self detecting abnormalities.
Programmable controllers are conventionally provided with abnormality diagnostic means for identifying abnormal battery voltages, abnormal temperature rises, and abnormalities in data, etc., by performing parity checks for bus lines. Conventional programmable controllers are constituted in such a manner that the action of the programmable controller is stopped (its output signal is turned off) when an abnormality in the device is detected by the abnormality diagnostic means.
However, the programmable controller, constituted as described above, in which operation of the device is fully stopped at the occurrence of any abnormality in the device, regardless of the kind of abnormality or the status of objective control at the occurrence of the abnormality, is not always practical from the standpoint of its user.
The above mentioned problem normally occurs in a set of programmable controllers. Similarly a further complicated problem occurs in a programmable controller system of the parallel link mode which has already been developed by the inventors of this invention. Referring to the above, a, description will be first given for the programmable controller system of the parallel link mode.
Though a large scale programmable controller must be conventionally used for large scale objective control, the programmable controller system of the parallel link mode has also been considered capable for application to large scale objective control by using plural sets of comparatively small scale programmable controllers. In short, each of the programmable controllers is provided with an input/output memory having larger capacity than a number of intra input/output terminals. For instance, when it is assumed that 64 I/O terminals are provided in a set of programmable controllers a further maximum four sets of controllers can be combined as a system of the parallel link mode. The I/O memory of each controller is provided with capacity of at least 64.times.4 bits as an I/O data area. When using only a set of programmable controllers, where they operate as a normal controller with 64 I/O items, I/O data exchange means and user program synchronously executing means are provided in each of the programmable controllers so that data of each I/O memory may be exchanged with each other synchronously with execution action of each user program when plural sets of controllers are combined by a link bus. In this way, control action is performed so that an I/O state is transmitted to a local machine, an I/O state of the local machine is received to an intra machine, and on the basis of the I/O states in those total units a user program is executed respectively in each machine to again transmit its execution result to each machine with each other. Accordingly, in a system linked with four sets of programmable controllers, as described above, a control function equal to that obtained in a series of large scale objective control is controlled by a set of programmable controllers with 64.times.4 I/O, and can be realized by partially executing a control program for the control function with four sets of controllers.
In case the above described system of parallel link mode, since plural sets of linked programmable controllers are allowed to function as a set of programmable controllers in the case of a normally uncontrollable state, at an occurrence of abnormality even in a set of plural linked controllers it is a very hazardous state in that with an objective control equipment it can sometimes result in a quite random controlled condition, unless control action of the system total unit is correspondingly performed for the abnormality.
Further the conventional programmable controller is provided with such abnormality diagnostic means that abnormal voltage in a battery is detected, that an abnormal temperature rise is detected in a device, that a parity check for a bus line in the device is performed to detect abnormality of data, etc., and is constituted in such a manner that the action of the programmable controller is stopped (an output signal is all turned off) when the abnormality of the device is detected by said abnormality diagnostic means.
However, the programmable controller, in which the action of a device is fully stopped regardless of the kind of abnormality or the status of objective control at the occurrence of the abnormality when any abnormal state is caused in the device, has been frequently in a state not always adequate for a practical case from the standpoint of its user.
The above described matter, for a system of parallel link mode, results in a further complicated problem, when abnormality is detected in each programmable controller, that is it is not adequate for its action to be fixedly stopped in a hardware manner.